In recent years, there has been increasing interest in energy storage technologies. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, notebook computers and even electric cars, there has been a growing demand for high energy-density batteries as power sources for such electronic devices. In response to this demand, research on lithium secondary batteries is being actively undertaken.
Many companies have produced a variety of lithium secondary batteries with different safety characteristics. It is very important to evaluate and ensure safety of such lithium secondary batteries. The most important consideration for safety is that operational failure or malfunction of lithium secondary batteries should not cause injury to users. For this purpose, safety regulations strictly restrict the possibilities of dangers (such as fire and smoke) of lithium secondary batteries.
Lithium-containing metal oxides and carbon materials are usually used at present as cathode and anode active materials of lithium secondary batteries, respectively. However, a lithium secondary battery fabricated using the electrode active materials suffers from safety problems, for example: overcharge/overdischarge resulting from malfunction of a protective device; internal short circuits caused by an external impact or defects of a separator; exothermic reactions between a charged cathode and an electrolyte solution under abnormal conditions, such as during high-temperature storage, to cause decomposition of the cathode; heat emission arising from short circuits between the cathode and an anode; and decomposition reactions of the electrolyte solution at the electrode interface.
In an effort to solve the above safety problems caused by electrode active materials, a technique has been proposed in which a metal oxide is coated on the surface of at least one electrode layer or the surfaces of electrode active material particles. This metal oxide coating is known to be effective in inhibiting a change in the volume of the electrode active material during charge and discharge and in blocking an electrolyte solution from being in direct contact with the electrode active material to prevent side reactions of the electrolyte solution.
On the other hand, as the charge/discharge cycles of a battery proceed, metal ions (typically transition metal ions) other than lithium are also dissolved from a lithium-containing metal oxide as a cathode active material. A metal oxide coating layer can inhibit dissolution of the metal ions to some extent but cannot completely block the electrode active material. Further, passages through which lithium ions pass are present in the metal oxide coating layer. Therefore, dissolution of the metal ions is unavoidable. The dissolved metal ions cause decomposition reactions of an electrolyte solution.
Along with the recent increasing demand for high-capacity lithium secondary batteries, high voltages are required as conditions for battery operation. However, high-voltage operating conditions primarily increase side reactions of electrolyte solutions and also induce rapid dissolution of metal ions from lithium metal oxides as cathode active materials. The dissolved metal ions further promote side reactions of electrolyte solutions.